Most, if not all, commercial trucking vehicles use a standard axle and bearing configuration to mount the wheels on the axle. The construction of the axle housing differs somewhat between whether the axle is mounted on a tractor, where a differential gear device is connected to a pair of oppositely extending hollow tubes to provide means to permit the transmission if power from the differential gear device to the wheels, or whether the axle is mounted on a semi-trailer where the axle is a solid body.
Regardless of the application, i.e. tractor or semi-trailer, the wheel assembly and method of mounting the wheel assembly on the axle generally follows a standard pattern and procedure.
The axle whether hollow (tractor) or (trailer) provides a base unit to which a brake shoe anchor bracket is mounted. A splash guard or backing plate for the braking assembly of the vehicle may also be mounted on the axle.
A pair of brake shoes are mounted on the brake shoe anchor bracket and are spring loaded to be normally in a retracted position. The brake shoes are usually activated by a brake camshaft which twists to force the ends opposite the mounting pivots of the brake shoes apart, so as to contact a surrounding rotating brake drum and apply a braking effort.
The brake drum is mounted on a rotor or hub which is mounted on the axle. In either case, the hub or drum is provided with a pair of spaced wheel bearings (usually tapered) which provide the anti-friction means to permit the wheel assembly to rotate with minimum runout and yet endure the forces imposed by gravity, braking, acceleration and shocks due to uneven road surfaces. At the same time, the lateral forces on the two bearings imposed by the twisting torque imposed on the wheel bearings of a trailer during the execution of a turn on a concrete or asphalt surface can be significant.
The hollow rotor or hub assembly is usually partially filled with a lubricating oil to provide a permanent method of lubricating the two wheel bearings. This means that both ends of the rotor must be provided with sealing means to prevent the escape of lubricating oil from the rotor cavity.
The axle itself is usually tapered so that the wheel bearings inside the rotor have markedly different diameters, such that the bearing having larger diameter of the two bearings is located inboard (nearer the differential gear device) and the smaller diameter bearing is outboard.
The inboard seals in the wheel assemblies are subjected to wear, aging, mechanical damage, intrusion of foreign matter in the seal-shaft interface which may cause damage to the sealing membrane of the seal. As a result, the oil level in the rotor hub may drop as the bearing oil gradually leaks past the inboard seal to escape from the hub cavity. As the oil leaks from the cavity, the usual occurrence will be that the outboard wheel bearing (smaller) will be the first bearing to undergo lubrication oil starvation, because it is at a higher level on the axle, (due to its smaller diameter). Usually, this condition will be found during routine maintenance operations, but if it is not detected, the failure of the outboard bearing is almost inevitable.
The two wheel bearings are usually held in compression on the axle by a double-nut-ring locking mechanism which applies compressive forces to the two inside races of the wheel bearings.
If the rollers of the outboard bearing disintegrate due to failure of the lubrication system, the rotor may allow the whole wheel assembly to leave the axle because the diameter of the aperture in the inboard outer bearing race is usually greater than the outer diameter of the inner race of the outboard bearing (which remains captive on the axle, because of the locking mechanism on the axle).
Thus, the failure of the inboard seal can give rise to a lubrication deficit which even though gradual, can lead to the condition where the outboard bearing fails and the wheel-brake drum-rotor assembly may leave the axle. If this occurs on a tractor, the driver may be alerted to the bearing failure by abnormal driving symptoms or the generation of excessive noise. If this malfunction occurs on an axle of a trailer at a distance of some forty feet from the driver during a freight delivery operation, the malfunction or absence of the wheel assembly may go undetected for a considerable distance before the driver of the transport vehicle becomes aware of the problem. As the tapered outboard bearing enters its failured mode, the hub is no longer held in a concentric relationship with the axle spindle, thus the hub moves upwardly and backwardly on the spindle as the bearing rollers begin to disintegrate. The device of this invention is adapted to sense this eccentricity and immediately alert the driver of a pending failure of a wheel bearing before the above described condition deteriorates to the situation where the whole wheel assembly (including brake drum) leaves the vehicle. This device will perform equally well on tractors or trailers.